The invention relates to a capacitive sensor device for detecting capacitance changes at a proximity sensor. When an object or body approaches a sensor electrode, the capacitive sensor device according to the invention and the associated evaluation circuit detect a change in the capacitance at the sensor electrode.
Capacitive proximity switches are known in the prior art in the field of application for vehicles. By way of example, EP 1505734 discloses a capacitive proximity switch and an associated evaluation process. Capacitive proximity switches of this type have sensor electrodes and should respond when a certain capacitance or capacitance change is detected between the sensor electrode and a reference electrode. For this, numerous electrodes are frequently disposed in the region of a vehicle that is configured for detecting operation.
A capacitance is formed between the sensor electrode, which is brought to a predefined potential, and a reference electrode, e.g. the vehicle ground or the ground of the surface located under the vehicle. This capacitance can change when a body moves in the sensitive region of the sensor assembly, e.g. the hand of a user, or the foot of a user. The detection of the capacitance itself can be achieved in a variety of ways. By way of example, a number of clocked discharge procedures or a discharge period can be used as the measurement value for the capacitance. Evaluation methods of this type are sufficiently known in the prior art. By way of example, a capacitive detection of a proximity to a vehicle door handle is known from DE 19617038A1.
One problem with the known sensor assemblies is that external factors lead to changes in the capacitance value of the sensor electrode, without the approach of a user. The capacitance of a sensor electrode is highly dependent on the environment, e.g. the air humidity, a coating with rain, snow, ice or dirt in the region of the sensor electrode. In this context, a capacitive proximity sensor is known, for example, from DE 19620059A1, which only switches when the capacitance between the response electrode and the reference electrode changes at a rate that is higher than a lower threshold value.
These concepts do not, however, change the fact that the evaluation circuit is normally configured to an operating point that corresponds to an expected capacitance that is to be evaluated, or to a change in the capacitance. If the capacitance is significantly changed by external factors, the precision of the evaluation also normally decreases, because the configuration of the circuit no longer corresponds to the base value of the capacitance.
The object of the invention is thus to provide an improved sensor assembly and evaluation circuit, which enables a capacitance change in an improved manner, even in environmental conditions that may change.
This object is achieved according to the invention by means of a device having the features of Claim 1.